Fat loss is all about energy balance. Almost everybody has heard that there’s 3,500 calories in a pound of fat, so if you create a negative energy balance of 3500 calories in a week, you lose a pound of weight. Create a negative energy balance of 7000 calories (deficit) in a week and you tip the scale to a two pound weight loss and so on, right? Not so fast… Dr. Kevin Hall, an investigator at the National Institute of Health in Bethesda has done some interesting research about the mechanisms regulating human body weight. He recently published a new paper in the International Journal of Obesity that throws a wrench in works of the “3500 calories to lose a pound” idea…

Some of the equations in his paper gave me a headache… but despite the complex formulas he used to come to his conclusions, the article contained a lot of simple and very practical tips you can use to properly balance your caloric intake with output, fine tune your calorie deficit and help you retain more muscle when you diet.

Below, I’ve distilled some of the information into a simple bullet-point summary that any non-scientist can understand, and then I wrap up with my interpretation of how you can apply this:

3500 calories to lose a pound has always been the rule of thumb. However, this 3500 calories figure goes back to research which assumed that all the weight lost would be adipose tissue (which would be ideal, of course).
But as we all know (unfortunately), lean body mass is lost along with body fat, which would indicate that the 3500 calorie figure could be an oversimplification.
The amount of lean body mass lost is based on initial body fat level and size of the calrie deficit
Lean people tend to lose more lean body mass and retain more fat.
Fat people tend to lose more body fat and retain more lean tissue (revealing why obese people can tolerate extremely low calorie diets better than already lean people)
Very aggressive low calorie diets tend to erode lean body mass to a greater degree than more conservative diets.
whether the weight loss is lean or fat gives you the real answer of what is the required energy deficit per unit of weight loss
The metabolizable energy in fat is different than the metabolizable eneregy in muscle tissue. A pound of muscle is not 3500 calories. A pound of muscle yields about 600 calories.
If you lose lean body mass then you lose more weight than if you lose fat.
If you create a 3500 calorie deficit in one week and you lose 100% body fat, you will lose one pound.
But if you create a 3500 calorie weekly deficit and as a result of that deficit, lose 100% muscle, you would lose almost 6 pounds of body weight! (of course, if you mangage to lose 100% muscle, you will be forced to wear the Dieter’s Dunce cap)
If you have a high initial body fat percentage, then you are going to lose more fat relative to lean, so you will need a larger deficit to lose the same amount of weight as compared to a lean person!
Creating a calorie deficit once at the beginning of a diet and maintaining that same caloric intake for the duration of the diet and after major weight loss fails to account for how your body decreases energy expenditure with reduced body weight
Weight loss typically slows down over time for a prescribed constant diet (the “plateau”). This is either due to the decreased metabolism mentioned above, or a relaxing of the diet compliance, or both (most people just can’t hack aggressive calorie reductions for long)
Progressive resistance training and or high protein diets can modify the proportion of weight lost from body fat vs lean tissue (which is why weight training and sufficient protein while on calorie restricted diets are absolute musts!)
So, based on this info, should you throw out the old calorie formulas?

Well, not necessarily. You can still use the standard calorie formulas to figure out how much you should eat, and you can use a 500-1000 calorie per day deficit (below maintenance) as a generic guideline to figure where to set your calories to lose one or two pounds per week respectively (at least on paper anyway).

Even better however, you could use this info to fine tune your caloric deficit using a percentage method and also base your deficit on your starting body fat level, to get a much more personalized and effective approach:

(Note: According to exercise physiologists Katch & Mcardle, the average female between the ages of 23 and 50 has a maintenance level of about 2000-2100 calories per day and the average male about 2700-2900 calories per day)

Usually, we would suggest starting with a conservative deficit of around 15-20% below maintenance. What Dr. Hall’s research is saying, is that there can be big differences between lean and overweight people in how many calories they can or should cut.

If you have very high body fat to begin with, the typical rule of thumb on calorie deficits may underestimate the deficit required to lose a pound. It may also be too conservative and you can also probably use a moderate to aggressive deficit more safely without as much worry about muscle loss, metabolic slowdown, etc.

If you are extremely lean, like a bodybuilder trying to get even leaner for competition, you would want to be very cautious using aggressive calorie deficits. You’d be better off keeping the deficit conservative and starting your diet/cutting phase earlier to allow for a slow, but safe rate of fat loss, with maximum retention of muscle tissue.

The long and short of it is that its not quite so simple as 3,500 calories being the deficit to lose a pound. Like lots of other things in nutrition that vary from person to person, the ideal amount of calories to cut “depends”…

I don't think so since he talks about 3500 calories of lean muscle lost equals 6 pounds. I don't really have an answer to your initial question, although water is the most logical answer, or possibly it might require a certain amount of evergy to break down muscle. I'm just speculating, i really don't know.

i've always questioned calories in general, as its a measurement of direct thermal energy vs chemical...

Yes we often get too focussed on the details and lose sight of the big picture. The body is NOT a simple machine and measuring the relevant values accurately (ie energy expenditure) can be difficult. IMO it is best to stay focussed on the fundamental concepts and use the numbers as guidelines.